1. Field of the Invention
The present invention relates to a radio receiver comprising an RF-AGC loop and having an automatic memory function.
2. Description of the Related Art
In general, when a radio receiver receives a disturbance station "fud" having strong field intensity, in addition to a station "fd" which the receiver intends to receive (an object station), a harmonic is generated according to the non-linear element characteristics of an element used in an RF amplifier, and so on. In particular, if two disturbance stations "fud1," "fud2" (fud1&lt;fud2) are present, and they hold the relationship "fd-fud1=fud1-fud2" with respect to an object station, as shown in FIG. 1, a harmonic due to the disturbance stations "fud1," "fud2," i.e., (2.times.fud1-fud2, fud2-2.times.fud1), will appear in the vicinity frequency of the object station. This harmonic may be transmitted via the receiver line to a detection stage, which may cause mutual modulation disturbance.
In order to solve this problem, conventionally, an RF signal level has been lowered according to received field intensity to thereby suppress generation of a harmonic and further to prevent occurrence of mutual modulation disturbance. FIG. 2 is a block diagram relative to a conventional radio receiver which is less affected by mutual modulation disturbance.
Referring to FIG. 2, a received RF signal of an object station is supplied to an RF amplifier 1 for amplifying, and then to an RF tuning circuit 2 for tuning. The RF tuning circuit 2 in turn outputs a signal into a mixing circuit 3, where the signal is converted through frequency conversion into an IF signal by a local oscillation signal supplied from a local oscillation circuit 4. The IF signal is supplied to an IF amplifier 5 for amplifying, and then to an FM detection circuit 6 for FM detection.
Then, if disturbance stations fud1, fud2 having strong field intensity are present, a field intensity detection circuit 7 outputs a high level signal into an RF-AGC circuit 8, so that the circuit 8 outputs a signal according to the field intensity. In response to this signal outputted from the circuit 8, the RF amplifier 1 controls its gain. Specifically, if the circuit 1 receives a signal higher than a predetermined level field intensity, the circuit 1 reduces its gain such that it reduces the gain more when it receives a signal indicating higher intensity.
In other words, when disturbance stations fud1, fud2 are received, the gain of the RF amplifier 1 is reduced, and the levels of the received RF signals of the disturbance stations fud1, fud2 are accordingly reduced until adverse effects by these disturbance stations fud1, fud2 are modified.
However, the presence of a disturbance station which has a close frequency to that of the object station, and does not hold the above mentioned relationship with an object station may lead to erroneous operation of the AGC loop. As a result, the level of an RF signal of the object station may be undesirably suppressed even when mutual modulation disturbance is not occurred.